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<div class="section-level-extent" id="Alignment-of-poly_005fints">
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Next: <a href="Computing-bounds-on-poly_005fints.html" accesskey="n" rel="next">Computing bounds on <code class="code">poly_int</code>s</a>, Previous: <a href="Arithmetic-on-poly_005fints.html" accesskey="p" rel="prev">Arithmetic on <code class="code">poly_int</code>s</a>, Up: <a href="poly_005fint.html" accesskey="u" rel="up">Sizes and offsets as runtime invariants</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Option-Index.html" title="Index" rel="index">Index</a>]</p>
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<h3 class="section" id="Alignment-of-poly_005fints-1"><span>10.5 Alignment of <code class="code">poly_int</code>s<a class="copiable-link" href="#Alignment-of-poly_005fints-1"> &para;</a></span></h3>

<p><code class="code">poly_int</code> provides various routines for aligning values and for querying
misalignments.  In each case the alignment must be a power of 2.
</p>
<dl class="table">
<dt>&lsquo;<samp class="samp">can_align_p (<var class="var">value</var>, <var class="var">align</var>)</samp>&rsquo;</dt>
<dd><p>Return true if we can align <var class="var">value</var> up or down to the nearest multiple
of <var class="var">align</var> at compile time.  The answer is the same for both directions.
</p>
</dd>
<dt>&lsquo;<samp class="samp">can_align_down (<var class="var">value</var>, <var class="var">align</var>, &amp;<var class="var">aligned</var>)</samp>&rsquo;</dt>
<dd><p>Return true if <code class="code">can_align_p</code>; if so, set <var class="var">aligned</var> to the greatest
aligned value that is less than or equal to <var class="var">value</var>.
</p>
</dd>
<dt>&lsquo;<samp class="samp">can_align_up (<var class="var">value</var>, <var class="var">align</var>, &amp;<var class="var">aligned</var>)</samp>&rsquo;</dt>
<dd><p>Return true if <code class="code">can_align_p</code>; if so, set <var class="var">aligned</var> to the lowest
aligned value that is greater than or equal to <var class="var">value</var>.
</p>
</dd>
<dt>&lsquo;<samp class="samp">known_equal_after_align_down (<var class="var">a</var>, <var class="var">b</var>, <var class="var">align</var>)</samp>&rsquo;</dt>
<dd><p>Return true if we can align <var class="var">a</var> and <var class="var">b</var> down to the nearest
<var class="var">align</var> boundary at compile time and if the two results are equal.
</p>
</dd>
<dt>&lsquo;<samp class="samp">known_equal_after_align_up (<var class="var">a</var>, <var class="var">b</var>, <var class="var">align</var>)</samp>&rsquo;</dt>
<dd><p>Return true if we can align <var class="var">a</var> and <var class="var">b</var> up to the nearest
<var class="var">align</var> boundary at compile time and if the two results are equal.
</p>
</dd>
<dt>&lsquo;<samp class="samp">aligned_lower_bound (<var class="var">value</var>, <var class="var">align</var>)</samp>&rsquo;</dt>
<dd><p>Return a result that is no greater than <var class="var">value</var> and that is aligned
to <var class="var">align</var>.  The result will the closest aligned value for some
indeterminate values but not necessarily for all.
</p>
<p>For example, suppose we are allocating an object of <var class="var">size</var> bytes
in a downward-growing stack whose current limit is given by <var class="var">limit</var>.
If the object requires <var class="var">align</var> bytes of alignment, the new stack
limit is given by:
</p>
<div class="example smallexample">
<pre class="example-preformatted">aligned_lower_bound (<var class="var">limit</var> - <var class="var">size</var>, <var class="var">align</var>)
</pre></div>

</dd>
<dt>&lsquo;<samp class="samp">aligned_upper_bound (<var class="var">value</var>, <var class="var">align</var>)</samp>&rsquo;</dt>
<dd><p>Likewise return a result that is no less than <var class="var">value</var> and that is
aligned to <var class="var">align</var>.  This is the routine that would be used for
upward-growing stacks in the scenario just described.
</p>
</dd>
<dt>&lsquo;<samp class="samp">known_misalignment (<var class="var">value</var>, <var class="var">align</var>, &amp;<var class="var">misalign</var>)</samp>&rsquo;</dt>
<dd><p>Return true if we can calculate the misalignment of <var class="var">value</var>
with respect to <var class="var">align</var> at compile time, storing the result in
<var class="var">misalign</var> if so.
</p>
</dd>
<dt>&lsquo;<samp class="samp">known_alignment (<var class="var">value</var>)</samp>&rsquo;</dt>
<dd><p>Return the minimum alignment that <var class="var">value</var> is known to have
(in other words, the largest alignment that can be guaranteed
whatever the values of the indeterminates turn out to be).
Return 0 if <var class="var">value</var> is known to be 0.
</p>
</dd>
<dt>&lsquo;<samp class="samp">force_align_down (<var class="var">value</var>, <var class="var">align</var>)</samp>&rsquo;</dt>
<dd><p>Assert that <var class="var">value</var> can be aligned down to <var class="var">align</var> at compile
time and return the result.  When using this routine, please add a
comment explaining why the assertion is known to hold.
</p>
</dd>
<dt>&lsquo;<samp class="samp">force_align_up (<var class="var">value</var>, <var class="var">align</var>)</samp>&rsquo;</dt>
<dd><p>Likewise, but aligning up.
</p>
</dd>
<dt>&lsquo;<samp class="samp">force_align_down_and_div (<var class="var">value</var>, <var class="var">align</var>)</samp>&rsquo;</dt>
<dd><p>Divide the result of <code class="code">force_align_down</code> by <var class="var">align</var>.  Again,
please add a comment explaining why the assertion in <code class="code">force_align_down</code>
is known to hold.
</p>
</dd>
<dt>&lsquo;<samp class="samp">force_align_up_and_div (<var class="var">value</var>, <var class="var">align</var>)</samp>&rsquo;</dt>
<dd><p>Likewise for <code class="code">force_align_up</code>.
</p>
</dd>
<dt>&lsquo;<samp class="samp">force_get_misalignment (<var class="var">value</var>, <var class="var">align</var>)</samp>&rsquo;</dt>
<dd><p>Assert that we can calculate the misalignment of <var class="var">value</var> with
respect to <var class="var">align</var> at compile time and return the misalignment.
When using this function, please add a comment explaining why
the assertion is known to hold.
</p></dd>
</dl>

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